1. Field of the Invention
This invention relates to a novel and improved process for the synthesis of urea, and more particularly, it relates to an improvement in a process wherein the separation of unreacted carbon dioxide and ammonia (hereinafter referred to as unreacted materials) from a urea synthesis effluent is effected under a pressure equal to urea synthesis pressures, and a gaseous mixture of the separated carbon dioxide and ammonia is recycled to urea synthesis for reuse.
2. Description of the Prior Art
In the case where the unreacted materials are separated from the urea synthesis effluent and recovered to be recycled to urea synthesis for reuse under pressures substantially equal to urea synthesis pressures, it is common practice that the separation of the unreacted materials is promoted by the stripping thereof with carbon dioxide. A gaseous mixture of the unreacted materials thus separated is recovered by condensing the gaseous mixture to form ammonium carbamate, and heat generated is removed by means of producing steam. Generally, the condensation of the gaseous mixture is not effected completely, so that after the step of removing heat generated the unreacted materials thus recovered are recycled to urea synthesis as a mixture of an ammonium carbamate solution and an uncondensed gaseous mixture. As described above, the gaseous mixture is not completely condensed, so that the uncondensed part of the gaseous mixture may be condensed in a urea synthesis autoclave to generate heat and heat balance in the urea synthesis autoclave may be maintained.
There are two proposed methods of recycling the unreacted materials thus recovered to the urea synthesis autoclave: a gravity flow metod, in which the unreacted materials thus recovered are allowed to flow downwards due to a gravity head thereof, and another method, in which a fluid other than the unreacted materials, for example, ammonia is pressurized to an elevated pressure, so that the high pressure ammonia may be used as a driving medium for operating an ejector, and the unreacted materials may be introduced into the urea synthesis autoclave by aspiration action of the ejector. The gravity flow method has such disadvantages as the need for a great difference in height between a position where the unreacted materials are recycled and a condenser for the unreacted materials, that it is not always possible to recycle the unreacted materials thus recovered, which consist of a mixture of a liquid phase and a gas phase a specific gravity different from each other, to the urea synthesis autoclave at constantly uniform rate. Another drawback is that unstable operation conditions are resulting among the urea synthesis autoclave, a separator for the unreacted materials or stripper, and a condenser for the unreacted materials. On the other hand, the method with the ejector has the disadvantage that as the ratio of gas phase to liquid phase in the phase mixture is increased, the driving fluid is required to be pressurized to an extremely high pressure so that the ejector might be able to operate, resulting in necessity for more power.
In the condenser for the unreacted materials, a portion of the unreacted materials is not condensed in order to maintain the heat balance in the urea synthesis autoclave as described above. However, the following processes are further proposed, in which the heat balance in the urea synthesis autoclave can be maintained even if no gaseous phase remains in the unreacted materials recovered in the condenser for the unreacted materials. According to one proposed process, a sufficient amount of make-up carbon dioxide for maintaining the heat balance in the urea synthesis autoclave is fed directly to the urea synthesis autoclave, and the balance is used for stripping the unreacted materials, and substantially all of the gaseous mixture of the unreacted materials thus obtained is condensed to be recycled as liquid to the urea synthesis autoclave for reuse. According to another process, the unreacted materials are separated in two stages under pressures equal to each other, that is, a gaseous mixture of the unreacted materials separated in a first stage is recycled directly to the urea synthesis autoclave, and substantially all of the gaseous mixture of the unreacted materials separated in a second stage is condensed to be recycled as liquid to the urea synthesis autoclave for reuse.
However, these two processes described above have the following problems: with respect to separation of the unreacted materials, according to the former process, separation of the unreacted materials will not always be effected as desired due to restrictions in the amount of carbon dioxide usable for stripping, and according to the latter process, it is necessary to keep a balance between the amount of the unreacted materials separated in the first stage and that in the second stage in order to obtain a gaseous mixture in the first stage in the proper amount required for maintaining heat balance in the urea synthesis autoclave, since there is a possibility of it becoming unstable particularly for operations in the second stage.